This invention relates to the art of induction heating and, more particularly, to method and apparatus for uniformly heating an elongated workpiece having a uniform cross-section over the major portion of its length with at least one enlargement disposed therealong.
The invention is particularly applicable to heating sucker rods which are employed in oil wells and the like and will be described with particular reference thereto. However, it will be appreciated that the invention has broader applications and may be used for inductively heating a variety of elongated workpieces having one or more areas of enlarged cross-section disposed therealong.
Sucker rods are utilized in the petroleum industry as a connecting length between a down hole oil well pump and the lifting or pumping device on the surface. Each rod is quite long, e.g., approximately 25-30 feet. A major portion of the rod has a uniform cross-section with an enlargement included adjacent each end thereof. These enlargements variously facilitate interconnecting a plurality of the rods in an end-to-end relationship. While there are a number of specific or detailed modifications which may be included in the sucker rod configurations of different manufacturers, almost all such rods have the foregoing general conformation and characteristics.
One of the manufacturing steps typically requires that sucker rods be heated to some predetermined temperature prior to passing through a subsequent processing operation. Such subsequent operations typically comprise an electrostatic spray chamber for having a coating of paint or plastic-like material applied to the rods. Since the rods need only be surface-heated for this purpose, induction-type heating finds particular advantage in this environment. However, because sucker rods do not have uniform cross-sections throughout the entirety of their lengths, it is necessary to in some way compensate for heating variations which otherwise would occur at the rod enlargement areas. Unless each rod is heated to a uniform temperature over the entire length, a subsequent processing operation may not be as effective as required. This is particularly true where a paint or plastic-like coating is to be applied.
A number of different arrangements to compensate for the above noted difficulty have previously been proposed, developed, and attempted. Two such arrangements are disclosed in the commonly assigned U.S. patent application Ser. Nos. 294,932 (filed Aug. 21, 1981) and 320,550 (filed Nov. 12, 1981). These two arrangements utilize variations in the inductor energization levels for different areas or sections of a workpiece, in particular, a sucker rod. While substantially improving the ability to uniformly heat workpieces of the type herein involved, these specific prior arrangements rely heavily on timers or other external sensing means for monitoring workpiece movement along a workpath and into an inductively coupled relationship with an inductor. Reliance on such external sensing means can result in some degree of workpiece heating variations due to problems in accurately determining the moment to effect an energization level change and/or due to inconsistencies in the workpiece feed rate.
It has, therefore, been considered desirable to develop an improved method and apparatus which will more reliably assure uniform induction heating of sucker rods and other workpieces of the general type described. The subject invention is deemed to meet these needs. Moreover, the invention is equally applicable to use in tempering, case hardening, through hardening, and normalizing of sucker rods or other workpieces.